1. Field of the Invention
The invention relates to the field of cryogenic hoses and in particular to cryogenic hoses which are insulated by a vacuum jacket and the method for creating vacuums in such hoses.
2. Description of the Prior Art
Cryogenic fluids are transferred through hoses for many applications in may different types of industries, including the nuclear industry. In many cases, the application in which the cryogenic fluid must be used is located at some distance away from the source of cryogenic fluid. Therefore, if effective transfer is to occur between the source and the site of application, the hose must be well insulated to avoid the absorption of ambient heat into the cryogenic fluid being transferred into the flowing cryogenic fluid and thereby loss or degradation of the cryogenic fluid.
Insulated cryogenic hoses often include some type of insulating filler or fiber between the inner tube which conveys the cryogenic fluid and an outer protective jacket. Such filler typically includes material having a high thermal resistivity such as urethane foam insulation in combination one or more heat shielding layers.
However, the thermal resistivity of even the best fillers is relatively low and limits the practical length that can be utilized for such an insulated hose. Far better thermal insulation is obtained by vacuum jacketing. Therefore, cryogenic hose has been developed in which the tube carrying the cryogenic liquid is surrounded by a vacuum. However, in order to maintain a high vacuum within a jacket within a flexible or semiflexible hose, which is subjected to sometimes rough handling, is extremely difficult.
In addition, plastic and various types of synthetic materials are highly desirable if the flexibility of the hose is to be maintained. While good vacuums can be obtained in metallic jacketed hoses, the weight and rigidity of such materials makes such cryogenic hoses difficult to handle in long runs where the hose must be manipulated during use. The use of plastics or synthetic materials would lighten the hose and render it substantially more flexible, but outgassing from plastic materials is an inherent quality of these types of materials which ultimately leads to degradation of the vacuum and loss of its insulating quality. As a result, cryogenic vacuum jacketed hose having synthetic casing material must be maintained under vacuum by continual pumping or at least frequently re-evacuated.
Therefore, what is needed is a design for vacuum jacketed cryogenic hosing which overcomes each of these defects of the prior art.